This invention pertains to improved polyester molding compositions and more particularly to those having improved surface characteristics.
A technical improvement that has made a significant contribution to commercial polyester molding technology is the use of low profile additives to reduce shrinkage during the curing reaction, and to thereby improve dimensional stability and surface smoothness. Low profile additives are thermoplastic polymers such as vinyl acetate polymers, polystyrene, acrylic polymers, and polycaprolactones. There are a number of theories that seek to explain the low profile or anti-shrinkage action of these polymers, but the one that seems to best explain the phenomenon is the following:
The low profile additive is at least partly soluble in the uncured polyester/styrene solution. As the polyester/styrene mixture crosslinks, the thermoplastic polymer becomes incompatible or less soluble and at least partly comes out of solution. This action causes a volume expansion that compensates for the shrinkage that occurs when the polyester/styrene mixture crosslinks.
The development of low-profile unsaturated polyester compounds has led to a wide acceptance of these materials by the transportation industry because of their good surface appearance, dimensional stability, physical properties, assembly consolidation and potential weight savings. However, as new applications developed standards have been raised making it desirable for even better surface appearance and the elimination of ripples and waviness that sometimes develop, particularly in relatively large appearance sensitive areas.
There is, therefore, a need to provide low profile unsaturated polyester compounds which afford improved surface appearance in the molded parts obtainable therefrom.
U.S. patent application Ser. No. 194,069 filed in the name of Kenneth E. Atkins on Oct. 16, 1980, now U.S. Pat. No. 4,374,215, and titled "Improved Polyester Moldings" describes a polyester molding composition comprising
(a) a polyester resin comprising the reaction product of an olefinically unsaturated dicarboxylic acid or anhydride and a polyol; PA1 (b) an olefinically unsaturated monomer that is copolymerizable with said polyester resin; PA1 (d) an effective amount of a second crosslinkable vinyl monomer having a reactivity ratio with styrene of greater than 1. PA1 The addition of component (d) in said U.S. patent application Ser. No. 194,069 is stated to result in moldings with improved surface characteristics. PA1 (a) a thermosetting polyester resin; PA1 (b) an olefinically unsaturated monomer that is copolymerizable with polyester resins; PA1 (c) a thermoplastic low profile additive for shrinkage control; and PA1 (d) a thermosetting epoxy resin containing at least one 1,2-epoxy group per molecule. PA1 (a) a thermosetting polyester resin, PA1 (b) an olefinically unsaturated monomer that is copolymerizable with the polyester resin; PA1 (c) a thermoplastic low profile additive for shrinkage control PA1 (d) an effective amount of a second cross-linkable vinyl monomer having a reactivity ratio with styrene of greater than 1, and PA1 (e) a thermosetting epoxy resin containing at least one 1,2-epoxy group per molecule.
(c) a thermoplastic polymer low-profile additive to control shrinkage; and
U.S. patent application Ser. No. 194,070 filed in the name of Kenneth E. Atkins et al. on Oct. 16, 1980, now abandoned, and titled "Improved Polyester Fiber Reinforced Molding Compounds" describes a composition comprising:
The addition of component (d) in said U.S. patent application Ser. No. 194,070 is stated to result in moldings having improved surface appearance.